Field of the Invention
The invention relates to a micromechanical component and a process for its fabrication.
With a view to ever faster and smaller circuit configurations, attempts are made to integrate an electronic circuit together with a micromechanical component, for example, a sensor or an actuator, into a single chip.
In T. Scheiter et al., xe2x80x9cFull integration of a pressure sensor system into a standard BiCMCOS-Processxe2x80x9d, Eurosensors XI, 11th European Conference on Solid State Transducers, Warsaw, Poland (1997) 1595, a pressure sensor is described that is produced in a standard BiCMOS process on a surface of a silicon substrate. To produce the pressure sensor, a doped region, which acts as a first capacitor electrode of a capacitor, is produced at the surface of the substrate. A field oxide 600 nm thick, which serves as a sacrificial layer, is produced over the doped region. A layer of polysilicon 400 nm thick is deposited over the sacrificial layer. Openings are produced in the polysilicon layer. Then, through these openings, part of the sacrificial layer is removed by etching, which produces a cavity under the polysilicon layer. The openings are closed by a further deposited layer. The further layer is then structured such that parts of the polysilicon layer that are disposed above the cavity are exposed. Parts of the polysilicon layer and parts of the further layer, which are disposed respectively above the cavity, form a membrane. In the region of the openings, the membrane has thickened portions, at which it is inflexible. The parts of the polysilicon layer that are parts of the membrane act as second capacitor electrodes of the capacitor. As a result of deflection of the membrane because of a pressure, the distance between the first capacitor electrode and the second capacitor electrode is changed, which changes the capacitance of the capacitor, which is a measure of the pressure. The size of the area of the exposed parts of the polysilicon layer, that is to say, a deformable region of the membrane, determines the stiffness of the membrane. The higher the pressure range to be measured, the smaller the deformable region should be. One disadvantage is that mechanical loading brought about by the deflection is substantially distributed only to deformable parts of the membrane. For pressures that are greater than about 20 bar, such a pressure sensor is not suitable because the mechanical loading of the deformable membrane regions is close to the fracture limit. Moreover, process fluctuations, such as lithography faults, have an unmanageably large influence on the stiffness of the membrane because of the small size of the deformable regions.
In G. Ehrler, xe2x80x9cPiezoresistive Silizium-Elementardrucksensorenxe2x80x9d [Piezoresistive silicon-element pressure sensors], Sensormagazin 1/92, 10, a pressure sensor is described in which the pressure is measured with the aid of the piezoresistive effect. Four diffusion regions are produced on a surface of a silicon substrate and are connected to form a Wheatstone bridge. A passivation layer is disposed over the diffusion regions. An opening is produced in a rear of the substrate that reaches as far as the diffusion regions. The depression forms a pressure chamber that, for absolute pressure sensors, is under vacuum and is closed from below. A layer of the substrate, in which the diffusion regions are disposed and which is disposed above the pressure chamber, acts as a membrane of the pressure sensor. A pressure that acts on the passivation layer deflects the membrane, which produces stresses in the membrane. Because of the piezoelectric effect, the stresses lead to changes in the electrical conductivity of the layer of the substrate and, therefore, to changes in the magnitudes of the resistances of the diffusion regions, which are a measure of the pressure. Such a sensor is suitable as a high-pressure sensor. However, the process outlay for producing such a sensor is very high, in particular, due to the processing of the silicon substrate both on the front and on the rear.
In H. Dudaicevc et al., xe2x80x9cA fully integrated surface micromachined pressure sensor with low temperature dependencexe2x80x9d, Transducers ""95 Eurosensors IX (1995) 616, a pressure sensor is described in which a cell includes a first electrode of a capacitor, which is implemented at a doped region on a surface of a silicon substrate. An insulating layer of silicon nitride is applied to the surface. A sacrificial layer of oxide is deposited over the insulating layer and is structured such that it fills a cavity to be produced and has a diameter of about 100 xcexcm. A thin oxide layer is then deposited and structured to make a spur of oxide laterally adjoin the sacrificial layer. Then, a layer of polysilicon is deposited and structured to cover the sacrificial layer and part of the spur. By etching the oxide selectively with respect to polysilicon and silicon nitride, the thin oxide layer and the sacrificial layer are removed, the spur acting as an etching channel. The cavity is formed under the polysilicon layer. The polysilicon layer is supported on the silicon nitride layer. To seal off the cavity, oxide is deposited and closes the etching channel laterally. To completely cover the etching channel, it is important that the thin oxide layer not be too thick. As a result, the etching process is slow and possibly incomplete. In addition, an etching medium used can be flushed out only with difficulty. A number of identical cells are disposed in an x-y grid and connected in parallel with one another.
It is accordingly an object of the invention to provide a micromechanical component and process for its fabrication that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that can be configured as a high-pressure sensor and, as compared with the prior art, can be fabricated with a low process outlay or with a higher process reliability.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a micromechanical component, including at least one cell having a cavity with a given vertical dimension, a membrane acting as an electrode of a capacitor of the at least one cell, the membrane homogeneously disposed with a substantially uniform thickness over the cavity, a counter-electrode of the capacitor disposed under the cavity, at least one etching channel laterally adjoining the cavity, the at least one etching channel having a vertical dimension equal to the given vertical dimension, and at least one closure adjoining the at least one etching channel from above and disposed outside the membrane.
The objectives of the invention are achieved by a micromechanical component that includes at least one cell having a membrane that acts as an electrode of a capacitor of the cell and that is disposed with substantially uniform thickness over a cavity in the cell. Disposed under the cavity is a mating or counter electrode of the capacitor. At least one etching channel laterally adjoins the cavity. The etching channel has a vertical dimension equal to a vertical dimension of the cavity. A closure adjoins the etching channel from above.
With the objects of the invention in view, there is also provided a process for fabricating a micromechanical component, including the steps of producing a sacrificial layer over a counter-electrode of a capacitor of a cell, structuring the sacrificial layer to fill a region of a cavity to be produced in the cell and to fill an etching channel laterally adjacent to the cavity, conformally applying an upper conductive layer over the sacrificial layer, producing an opening into the upper conductive layer through the etching channel reaching as far as the sacrificial layer, etching the sacrificial layer to produce the cavity, a part of the upper conductive layer disposed over the cavity becoming capable of being deflected and acting as a membrane of the cell and as an electrode of the capacitor, and closing the etching channel in a region of the opening from above with a closure.
The objectives of the invention are achieved by a process for producing a micromechanical component in which a sacrificial electrode is produced over a mating or counter electrode of a capacitor of a cell of the component and is structured such that it fills a region of a cavity to be produced in the cell and has an etching channel laterally adjacent to the cavity. Conductive material is applied conformally over the sacrificial layer. Above the etching channel, an opening is produced that reaches as far as the sacrificial layer. The sacrificial layer is removed in an etching step, which produces the cavity, and a part of the conductive material disposed above the cavity becomes capable of being deflected and can act as a membrane of the cell and as an electrode of the capacitor. The etching channel in the region of the opening is closed from above by a closure. The process can be compatible with CMOS process technology.
The micromechanical component is based on the capacitive measurement principle. At least part of the cavity is part of a capacitor dielectric that is configured as an air gap and belongs to a capacitor whose capacitance is a measure of a pressure on the membrane. As such, the micromechanical component may be used, for example, as a pressure sensor or as a microphone.
Because a sacrificial layer is provided, the removal of which produces the cavity, the micromechanical component can be disposed on a surface of a substrate. As such, processing of a rear of the substrate is not necessary. Accordingly, the process outlay is lower when compared with a pressure sensor according to the Ehrler document. As opposed to the pressure sensor according to the Scheiter document, the membrane does not have any particularly thin points that could fracture particularly easily as a result of pressure. Nor does the membrane have any thickened portions where local load peaks would be produced at edges thereof that could lead to fracture of the membrane. The pressure is distributed uniformly over the membrane because the membrane is substantially uniformly thick. As a result, the membrane is more stable than the membrane according to the Scheiter document. During the removal of the etching medium used to etch the sacrificial layer, high capillary forces act on the membrane. If the membrane has particularly thin, that is to say soft, membrane parts, as in the case of the pressure sensor according to the Scheiter document, then they can remain stuck to the bottom of the cavity. Therefore, a further advantage of the uniformly thick membrane is that the membrane is mechanically very stable, even during the fabrication process.
The large vertical dimension of the etching channel is not an obstacle to sealing the cavity because the closure adjoins the etching channel from above. Such a feature is in contrast with the pressure sensor according to the Dudaicevc document.
The etching process proceeds more quickly and an etching agent can be flushed out more easily. As compared with the pressure sensor according to the Dudaicevc document, the micromechanical component may be produced with higher process reliability.
The sacrificial layer can be removed without having to produce an opening in the membrane because an etching channel is provided that laterally adjoins the sacrificial layer. Therefore, the membrane can be produced homogeneously and with a substantially equal thickness. The closure is located outside the membrane.
In accordance with another feature of the invention, the membrane can be part of an upper conductive layer that adjoins the cavity from above and laterally. As such, the sacrificial layer is applied to the entire area and then structured by masked etching. The upper conductive layer is then deposited to cover the sacrificial layer and surround it laterally. Apart from a region in which the etching channel adjoins the cavity, the upper conductive layer surrounds the cavity.
Alternatively, the cavity is surrounded laterally by a different material. For example, the sacrificial layer can be produced in a depression in the other material. Alternatively, the sacrificial layer is structured by masked etching and then the other material is deposited and planarized. The upper conductive layer is applied thereover.
The mating electrode can be implemented as a doped region in the substrate.
In order to avoid p-n junctions and associated depletion-layer capacitances and voltage restrictions, it is advantageous if the mating electrode is a first part of a structured lower conductive layer that is disposed on a first insulating layer that, in turn, is disposed on the surface of the substrate. The first insulating layer expediently has a thickness at which a capacitance formed by the mating electrode and the substrate is kept small.
In accordance with a further feature of the invention, there are provided a substrate having a surface, a first insulating layer being disposed on the surface, and a structured lower conductive layer having a first part and a second part, the lower conductive layer being disposed on the first insulating layer, the first part being the counter-electrode, and the upper conductive layer being supported on the second part of the lower conductive layer and being insulated from the first part of the lower conductive layer.
The upper conductive layer is preferably supported on a second part of the structured lower conductive layer. The second part of the structured lower conductive layer acts as an etch stop during the structuring of the sacrificial layer. The second part of the structured lower conductive layer is separated from the mating electrode such that the membrane, acting as the electrode, is not electrically connected to the mating electrode. The mating electrode can broaden underneath the etching channel, but otherwise occupies a smaller area than the cavity; that is to say, a horizontal cross section of the mating electrode in the region of the cell is smaller than a horizontal cross section of the cavity. For the capacitance of the capacitor of the cell to be as high as possible, the mating electrode occupies the greatest possible area. In other words, the area occupied by the mating electrode has the same shape as the area of the cavity and reaches virtually up to the edges of the cavity.
It is expedient if the first insulating layer and the sacrificial layer are made of the same material, for example, SiO2.
In accordance with an added feature of the invention, there is provided a second insulating layer disposed on the lower conductive layer with the cavity bounded from below by the second insulating layer.
In accordance with yet a further mode of the invention, a first insulating layer is produced on a substrate. A lower conductive layer is deposited over the first insulating layer and the lower conductive layer is structured to produce a first part of the lower conductive layer forming a counter-electrode and to produce a second part of the lower conductive layer separated from the first part. A second insulating layer is produced over the first conductive layer. The sacrificial layer is produced over the second insulating layer. The sacrificial layer is to be etched selectively with respect to the second insulating layer. The second insulating layer and the sacrificial layer are analogously structured to have the second insulating layer completely cover the first part of the lower conductive layer with the second part of the lower conductive layer acting as an etch stop. The upper conductive layer is produced over the sacrificial layer to partly adjoin the upper conductive layer laterally with the sacrificial layer.
If the sacrificial layer and the first insulating layer are made of the same material, it is advantageous if the sacrificial layer does not directly adjoin the first insulating structure in the region between the mating electrode and the second part of the structured lower conductive layer because the first insulating structure will otherwise be attacked as well. It is expedient to produce over the structured first conductive layer a second insulating layer, which can be etched selectively with respect to the sacrificial layer and with respect to the first insulating layer and over which the sacrificial layer is produced. The second insulating layer is structured in an analogous way to the sacrificial layer. In the region between the mating electrode and the second part of the structured lower conductive layer, the second insulating layer directly adjoins the first insulating layer and protects the latter during the removal of the sacrificial layer.
An electrical connection to the membrane can be made outside the cell through the second part of the structured lower conductive layer.
In accordance with an additional feature of the invention, the membrane has a substantially circular cross section, the at least one etching channel is at least three etching channels disposed at equal distances from one another, the at least one cell is a plurality of identical cells, respective groups of three of the cells adjoining one another have respective centers lying at corners of an equilateral triangle, and three etching channels of three different cells meet one another in a region over which the closure is disposed, the closure disposed between the three different cells.
It is advantageous if the membrane has a substantially circular cross section. As opposed to angular cross sections, the pressure is distributed more uniformly over the membrane to make the membrane more stable.
For exerting an external pressure over a large area, it is advantageous if the micromechanical component has a number of identical cells. Cavities in the cells are preferably connected to one another through the etching channels so that air pressures in the cavities substantially coincide. The connection also makes easier the diagnosis of membrane damage.
The closer the closures lie to one another, the easier the sacrificial layer may be removed. Consequently, it is advantageous if a diameter of the membranes is small and, for example, is between 10 xcexcm and 30 xcexcm. The size also results in a particularly high stability of the membranes. Because, in the case of a small diameter, the membranes are more difficult to deflect, the micromechanical component is suitable as a high-pressure sensor, particularly, as a high-pressure sensor for measuring pressures over 40 bar.
It is advantageous to configure the cells as closely to one another as possible because, with the same area of the micromechanical component, the sensitivity of the micromechanical component is increased as a result. If the membrane has a circular cross section, then the cells are preferably disposed in a hexagonal grid. In each case, three of the cells adjoin one another, with their centers lying at corners of an equilateral triangle. Consequently, a cell adjoins six cells disposed annularly around the cell. The cell can have three etching channels that are at equal distances from one another. In each case, three etching channels of different cells meet one another in a region over which the closure is disposed and which lies between these cells.
Instead of adjoining one another, the cells can be spaced apart from one another.
The cell can also have a number of etching channels differing from three.
In accordance with yet another feature of the invention, the mating electrodes of the cells can cohere through conductive webs running in the etching channels and, together with the webs, can form a first part of the structured first conductive layer. The first part of the structured lower layer acts as a common mating electrode of the cells.
The pressure range of the pressure sensor can be determined through the choice of the thickness of the membrane and the size of the horizontal cross section of the membrane. For a pressure range between 40 bar and 200 bar, the radius of the membrane is preferably between 13 xcexcm and 7 xcexcm.
The thinner the sacrificial layer, the greater is the change in the capacitance as the pressure changes. The thickness of the sacrificial layer is preferably between 200 nm and 500 nm.
The closure is produced, for example, by depositing and fusing boron phosphorous silicate glass (BPSG).
A number of layers of different materials can also be deposited in order to produce the closure or the upper conductive layer.
The lower conductive layer can be produced at the same time as upper capacitor electrodes of capacitors at the periphery of the micromechanical component or of capacitors in other parts of the substrate.
The first insulating layer and the sacrificial layer contain SiO2, for example. However, the use of other insulating materials lies within the scope of the invention. The second insulating layer can contain silicon nitride, for example. However, the use of other insulating material is likewise within the scope of the invention. The upper and the lower conductive layer contain, for example, doped polysilicon or another conductive material that, for example, contains metal. The SiO2 can be grown thermally or deposited. It is advantageous to apply a coating layer of silicon nitride or titanium nitride over the membrane, which protects the membrane against environmental influences.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a micromechanical component and process for its fabrication, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.